Principal Fault Research Articles

Normal fault architecture, evolution, and deformation mechanisms in basalts, Húsavik, Iceland: Impact on fluid flow in geothermal reservoirs and seismicity

Faults within layered basaltic sequences significantly influence hydrothermal fluid flow in shallow geothermal reservoirs and potentially during CO2 sequestration and storage. Nevertheless, their characterization regarding fault zone architecture, fluid flow, deformation mechanisms, and seismic potential remains underdeveloped. This study addresses this gap by integrating structural and microstructural observations with X-ray diffraction analyses of exposed normal-transtensional faults associated with the seismically active Húsavík-Flatey Fault in the Tjörnes Fracture Zone, Northern Iceland. Our findings demonstrate that the evolution of basalt-hosted normal-transtensional faults progresses through distinct stages: (1) low-displacement fault propagation from pre-existing cooling joints; (2) fault linkage via dilational jogs; (3) damage zone/fault core growth through brecciation and cataclastic processes; (4) shear localization along sharp slip surfaces; and (5) smearing of volcaniclastic interbeds along the principal fault plane. Evidence of shear localization, truncated clasts, and hydrothermal breccias/veins suggests repeated seismic slip events facilitated by overpressured fluids. Conversely, the presence of clay-rich foliated cataclasite indicates aseismic slips during interseismic periods. Slip along fault jogs, bends, geometric irregularities, and orientation changes causes the dilatant opening of the fault planes and extensional horsetail fractures at fault tips. These structures create main tabular zones for lateral movement of hydrothermal fluids parallel to the fault strike in shallow geothermal reservoirs situated in active extensional-transtensional tectonic settings. In addition, the dilational jogs and the intersection of horsetail veins with the hosting faults may define linear zones of high structural permeability and intense localized fluid flow parallel to the σ2 paleostress orientation and finally mineral precipitation. The results of this study can be utilized to improve models of geothermal fluid flow for enhanced recovery in basaltic reservoirs and assess seismic risk in basaltic faults.

Improved Simplified Engineering Fault Displacement Hazard Evaluation Method for On-Fault Sites

The safety of high-potential risk facilities concerned with fault displacement hazards is a complex technical issue, especially if the fault is revealed beneath the facility during the operation. Applying simple conservative engineering hazard evaluation methods is rational if an urgent decision should be made to continue operation or implement protective measures. Engineering methods are being published for strike-slip on-fault sites and structures. Their crucial point is to estimate the probability of the rupture at the site intersection and consider the displacement distribution over the rupture length relative to the site’s on-fault location. It is shown in the paper that strict geometrical relations between the site location, length and initial point of the rupture determine whether the principal fault displacement intersects the site. The paper considers these geometrical parameters as independent random variables and applies a screening of ruptures contributing to the hazard. Magnitude- and on-fault coordinate-dependent empirical relations have been analysed and selected to evaluate the site displacements. The procedure resulted in realistic but conservative hazard curves for different on-fault site locations using data from the Paks site in Hungary. The results were compared to those obtained by the conservative engineering method for the same site and some published analyses.

Statistically significant difference between earthquake size distributions of independent and triggered seismicity

The Alto Tiberina Fault system, located in Central Italy, is an active structure about 60 km long composed of a principal low-angle normal fault and several minor synthetic and antithetic splays. The system is monitored by a dense seismic network, giving us the opportunity to construct high-definition seismic catalogs with a low completeness magnitude. We analyze the clustering properties of the 2010-2015 seismicity by using a 3D stochastic declustering algorithm that also includes the earthquakes’ depth. We demonstrate that the earthquake size distribution is strongly correlated with the clustering of seismic events and their depth; in particular, the principal fault and secondary faults show an opposite behavior both in terms of earthquake size distribution and clustering properties.

Compound fault diagnosis of rolling bearings based on AVMD and IMOMEDA

The intricate nature of compound fault diagnosis in rolling bearings during nonstationary operations poses a challenge. To address this, a novel technique combines adaptive variational mode decomposition (AVMD) with improved multipoint optimal minimum entropy deconvolution adjustment (IMOMEDA). The compound fault signal is isolated through AVMD, with internal parameters obtained via a new indicator termed integrated fault-impact measure index guiding the improved dung beetle optimizer. An adaptive selection method, using a weight factor, chooses the intrinsic mode function containing principal fault data. IMOMEDA whose key parameters are determined by a novel combinatorial strategy is then employed to deconvolute selected fault components, enhancing periodic fault impulses by removing complex interferences and ambient noise. The deconvoluted signal undergoes enhanced envelope spectrum processing to extract fault frequencies and identify fault types. Numerical simulations and experimental data confirm the method’s effectiveness and feasibility for compound faults diagnosis of rolling bearings, showcasing its superiority over existing techniques.

Low-amplitude brittle deformations revealed by UAV surveys in alluvial fans along the northwest coast of Lake Baikal: Neotectonic significance and geological hazards

Many large deltas and other areas, underlain by unconsolidated sediments, are heavily populated but impacted by various natural deformational processes. The causes and mechanisms of the deformation are often obscure because of difficulties in the identification of their geological source. We used an unmanned aerial vehicle (UAV) to survey an area of up to several km2 to detect and map surface discontinuities with displacements of a few centimeters, which can indicate the presence of initial geological deformations. We encountered such offsets in the alluvial fans on the northwestern coast of Lake Baikal, overlying the damage zones of several faults. These deformations at the different sites are referred to as primary or secondary co-seismic ruptures, brittle creep, or cryogenic fissures, which have specific origins. They are located a few hundred meters from a principal fault plane, from junctions of faults, and in the peripheral parts of alluvial fans, making these areas potentially hazardous. We analyzed the digital surface models of local segments of one of the delta plains between 2020 and 2021 and established that its periphery has subsided by an average of 5–10 cm. In areas of accumulation of the river sediment, the thickness of coarse clastic deposits has increased by approximately the same amount. Locally and in the coastal zone, the vertical surface changes are larger. In the axial parts of some seismically induced gravitational failures, the subsidence reached 33 cm over a period of 11 months and 19 days. Our results show that sediments of alluvial fans are very susceptible to various tectonic and exogenous deformational processes. The interpretation of the ultra-high resolution UAV's images can help the recognition the low-amplitude brittle deformations at an early stage of their development. Therefore, such UAV surveys are critical in the discernment of neotectonic activity and its related hazards over short observation periods.

Slip localization on multiple fault splays accommodating distributed deformation across normal fault complexities

Deformation across structural complexities such as along-strike fault bends may be accommodated by distributed faulting, with multiple fault splays working to transfer the deformation between two principal fault segments. In these contexts, an unsolved question is whether fault activity is equally distributed through time, with multiple fault splays recording the same earthquakes, or it is instead localized in time and space across the distributed faults, with earthquakes being clustered on specific fault splays. To answer this question, we studied the distributed deformation across a structural complexity of the Mt. Marine fault (Central Apennines, Italy), where multiple fault splays accommodate the deformation throughout the change in strike of the fault. Our multidisciplinary (remote sensing analysis, geomorphological-geological mapping, geophysical and paleoseismological surveys) study identified five principal synthetic and antithetic fault splays arranged over an across-strike distance of 500 m, all of which showing evidence of multiple surface-rupturing events during the Late Pleistocene-Holocene. The fault splays exhibit different and variable activity rates, suggesting that fault activity is localized on specific fault splays through space and time. Nonetheless, our results suggest that multiple fault splays can rupture simultaneously during large earthquakes. Our findings have strong implications on fault-based seismic hazard assessments, as they imply that data collected on one splay may not be representative of the behaviour of the entire fault. This can potentially bias seismic hazard calculations.

Displacement Hazard from Distributed Ruptures in Strike-Slip Earthquakes

ABSTRACTWidespread distributed fracturing during earthquakes threatens infrastructure and lifelines. We combine high-resolution rupture maps from the five major surface-rupturing strike-slip earthquakes in southern California and northern Mexico since 1992 to incorporate the displacements produced by distributed ruptures into a probabilistic displacement hazard analysis framework. Through analysis of the spatial distribution of mapped ruptures and displacements for each of these events, we develop a magnitude-dependent expression for the probability per unit area of finding a distributed rupture that accommodates a displacement that exceeds a displacement threshold at a given distance from the principal fault. Our model is best applied to estimating expected distributed displacements for strike-slip earthquakes, similar to those analyzed, with widespread ruptures across immature fault zones.

Investigations of Weldment Joints of Exhaust Pipes Using Non-Destructive Testing (NDT)

Exhaust pipe components are especially vulnerable to accelerated degradation. Cracks are identified during the scheduled inspection and repaired by weld-filling procedure or by more invasive approaches like cutting and replacing sections. Such small defects can develop into fatigue or stress corrosion cracks in service, which can be extremely difficult to detect. The input parameter has a significant impact on weldment joint quality. Welds are checked to determine whether or not they meet specifications. The study was carried out to inspect and assess the quality of the weldment joints of exhaust pipes using non-destructive testing (NDT). An experimental approach methodology was adopted to achieve the aim of the study. Specimens of car exhaust pipes were welded and tested using some selected non-destructive testing techniques. Six (6) samples were selected for testing, with a serious fault or discontinuity in the artisan-made welded seam. The results showed that the principal fault signs were lack of fusion, surface porosity, crater fractures, linear indication, and undercuts, all of which were below the allowable standards of the AMSE B31.3 Stainless-Steel pipe code and therefore must be rejected.

Palaeostress state around a rising salt diapir inferred from seismic reflection data

A 3D seismic volume and borehole data from the Dutch North Sea are used to investigate the palaeostress state around a rising salt diapir. The results show radial, polygonal and keystone faults around the diapir of interest, which are separated into eight zones based on their geometry, strike and overall distribution. Principal fault families include: a) 400–3500 m long radial faults developed in flanking and corner areas of the salt diapir, and known to have accommodated the stretching resulting from its rise, b) 200–1500 m long polygonal faults providing a record of the stress conditions farther from radial faults, and c) 1000–1800 m long keystone faults controlled by the rise of a buried salt pillow during Paleogene tectonic inversion. Stress inversions for 10,401 interpreted faults reveal that the maximum principal palaeostress (σ1) is close to vertical, whereas intermediate and minimum principal palaeostresses (σ2 and σ3) are sub-horizontal, though variable in their magnitudes and orientations. Palaeostresses in flanking zones of the salt diapir show marked differences when compared with corner zones, but the combination of minimum principal palaeostresses from all flanking and corner zones formed a triangular stress ring around the salt diapir. Importantly, the width of this stress ring was not only associated with the rise of the salt diapir, but also largely influenced by adjacent salt structures. Additionally, minimum principal palaeostresses estimated from polygonal fault systems are nearly normal to the boundary between polygonal and radial faults, implying that the rise of the salt diapir influenced the stress field in the outer part of its flanks. Stress inversion is shown here as a key method to understand the palaeostress state around the salt diapirs with triangular geometry and multiple growth stages, providing insights into their evolving stress states. This work also has implications to the analysis of salt diapirs, and their adjacent strata, in areas posed for carbon sequestration, gas storage, and the production of oil and gas.

Interpreting the magnetic signatures and radiometric indicators within Kogi State, Nigeria for economic resources

The mineral and geothermal potentials of part of Kogi State was investigated through the interpretation of aeromagnetic and radiometric data of the study area. The analysis targeted bridging the gap of insufficient geophysical information of sub-crustal resources that could be of economic value within the area of study. Vertical derivatives, analytical signal and spectral depth analysis were used for the interpretation of the aeromagnetic data while the concentration and ternary images of the three radiogenic elements were used for the interpretation of the radiometric data. The result of first vertical derivative was helpful in delineating mineral potent lineaments labelled F1 to F8. The lineaments were seen trending E-W and NE-SW direction. A principal fault line F6 tends to separate the regions of sediments to the south-east and basement geologic formations to the south-western regions. Result of the analytical signal amplitude revealed regions with shallow intrusive magnetic rocks having high amplitudes ranging from 0.152 to 0.557 nT/m, while regions with magnetic rock intruding into sedimentary formations at greater depths, have medium to low amplitudes ranging from 0.014 to 0.136 nT/m. Regions delineated to be altered through hydrothermal process coincided with areas of major magnetic lineaments. The lineaments which could be fractures, faults or shear zones usually serve as conduits for mineral deposits during hydrothermal process. Result of potassium_thorium ratio map showed evidence of hydrothermal alteration in the NW and SW regions of study area. These regions of alterations also corresponds to regions where major lineaments were mapped and thus represent regions with significant potency for mineralisation. Result of spectral depth analysis on the aeromagnetic data showed that peak values of geothermal gradient and Heat flow were 27°C/km and 68 mW/m2 respectively, the values were recorded at the North-eastern part of study area, at the lower end of Koton-Karfe and part of Lokoja where the shallowest Curie point depth of 24 km also occurred. The estimated values of heat flow falls below the range of 80 to 100 mW/m2 recommended as threshold for a good source of geothermal energy, hence the region will not be prospectively good for a cost effective geothermal energy exploration. In order to assess the heat production within the study area due to activities of radioelements, a comparative analysis of the concentration of the three radiogenic elements was carried out. A relatively high radiogenic heat production (RHP) value of 3.4 μW/m3 was recorded at the North-western region indicating high occurrence of radioactivity within the granitic rocks. The (RHP) value of 3.4 μW/m3 is slightly below 4.0 μW/m3 which is a recommended value for a good source of geothermal energy.

Simulation of Dynamic Rupture Process and Near-Field Strong Ground Motion for the Wenchuan Earthquake

ABSTRACT The 2008 Wenchuan earthquake that occurred in the Longmenshan thrust zone is the most serious natural disaster recorded in China’s densely populated areas over the past few decades. Its northeast-trending principal fault—the Yingxiu–Beichuan fault (YXBCF), has a complex, segmentary-cascaded geometry and was dominated by the thrust slip in the southwest section, while the right-lateral slip in the northeast section. Some previous works believe that there may have occurred a supershear rupture in the strike-slip-dominated northeast section. Here we revisited this earthquake by exploring the dynamic rupture mechanism of the principal fault and showed a hypothetical scenario with supershear rupture occurring in its northeast section. We utilized a 3D curve grid finite-difference method to simulate the spontaneous rupture process of the YXBCF and corresponding near-field strong ground motion. An appropriate focal process is obtained using the trial-and-error method within reasonable parameters, and its related responses are validated by geological investigations and geophysical inversions. Besides, a large hypothetical oblique propagating supershear rupture is shown between Beichuan and Nanba in the northeast section of the YXBCF. Its transition mechanism is related to the Gaochuan–Beichuan conjugated fault from the fault geometry perspective, and belongs to the joint action of fault barrier and free surface. Such a supershear scenario is not rejected by observations and could increase the credibility of the occurrence of supershear rupture in the northeast section of the YXBCF during the Wenchuan earthquake.

Imbricated structure and hydraulic path induced by strike slip reactivation of a normal fault

In carbonates, fault zones strongly influence local reservoir properties and act as drains or barriers to flows. In the case of reactivated faults, newly developed fracture and fault systems rework and cross-cut pre-existing fault structures. The modifications of initial damage zone and fault core properties alter the fault zone properties and potentially created new fluid pathways. This study of the polyphase Castellas fault zone, developed as normal and later reactivated as strike slip, characterizes the structural pattern related to tectonic reactivation, and estimates the hydraulic behaviour of the fault zone through time. This fault affects carbonates located on La Fare anticlinal (SE France). High resolution mapping, 848 measurements and cross-sections allowed to discriminate the fault structures and the lateral heterogeneity of its architecture. Along the 850m-long studied damage zone we identify 8 principal fault planes, 2 relay zones, 19 secondary fault planes, and fault lenses. The study shows that the fault zone developed as isolated fault segments during the normal activity and acted as a drain to fluid flows during its nucleation. Later strike-slip reactivation led to the main structures visible presently as a well-connected fracture network and un-cemented breccia allowing the fault to be a drain to fluid flows.

Evolution of a Fault-Controlled, Deep-Water Sub-Basin, Tabernas, SE Spain

The Neogene Tabernas Basin, SE Spain, provides important evidence at outcrop for the interplay between tectonic deformation of the sea floor, slope instability and turbidity current behaviour. Dextral-oblique strike-slip faults and associated folds propagated along the basin axis to deform the palaeo-sea floor, creating structurally-controlled depressions in which turbidity currents were trapped and ponded. EW-trending syn-depositional faults define a narrow sub-basin that subsided asymmetrically as a negative flower structure. The sub-basin contains an expanded succession (>300 m of ponded turbidite sheets, debrites and slumps) along its northern margin flanked by the principal fault strand defined by a wide zone of sheared and calcite-veined marl. A narrower fault zone with a smaller displacement marks the southern margin of the sub-basin and the fill close to it is thin with internal discordances, evidence of local failure and southward thinning of sandstone sheets. Both northern and southern faults ‘died’ at the same stratigraphic level and were overstepped by basin floor turbidites showing evidence of weaker and longer-range topographic confinement. As turbidites healed and aggraded out of the sub-basin to progressively onlap the southern margin of the basin, major gravity failures occurred emplacing thick (>100 m) mass-transport complexes. The first initially reworked the southern part of the sub-basin fill together with the early onlap wedge, the second remobilised the onlap wedge, and the third records failure of the upper part of the slope well above the wedge. The first two were toe-confined failures, the third and furthest travelled was confined by basin axis topography. All three failures are lateral to or directly overlain by ‘megabed’ sheets on the basin floor, implying either a common trigger (earthquakes) or slope instability following reflection of large volume flows. Tabernas turbidites highlight the role of basin tectonics (as opposed to up-dip supply and sea level fluctuations) in directly impacting on deep-water processes and stratigraphy. Small deep-water transtensional sub-basins opened up along long transfer faults accommodating regional extension.

Probabilistic fault displacement hazard analysis of the Anghiari - Città di Castello normal fault (Italy)

Fault displacement is a localized source of hazard for infrastructure located near active and capable faults, such as critical buildings and distributed facilities. This issue can be overcome through zoning and avoidance strategies, but facilities may sometimes not benefit from this option. In the last 20 years, several authors have developed a probabilistic approach to analyse this hazard, known as probabilistic fault displacement hazard analysis (PFDHA), for both principal fault and distributed rupturing. In this work, we apply the approach proposed by YOUNGS et alii (2003) to the Anghiari-Città di Castello fault, a NE-dipping normal fault located in the northern Apennines of Italy, which shows evidence of Quaternary activity. We explore two different rupturing scenarios to obtain displacement hazard curves and maps for different probabilities of exceedance in 200 years at different distances along and from the principal fault trace for both the principal and distributed rupturing. The expected displacement values for the principal fault rupturing are up to 130 cm for a return period of 10,000 years (2% in 200 years), and the distributed rupturing hazard is 3–4% of the principal fault rupturing hazard. It is null in the footwall of the fault and slightly variable in its hanging wall as a function of distance from the fault trace and fault tip. The variability of the distributed rupturing hazard within the first hundreds of metres from the principal fault can assist to define mitigation strategies for existing facilities located in proximity of the fault.

Conditional probability of distributed surface rupturing during normal-faulting earthquakes

Abstract. Coseismic surface faulting is a significant source of hazard for critical plants and distributive infrastructure; it may occur either on the principal fault or as distributed rupture on nearby faults. Hazard assessment for distributed faulting is based on empirical relations which, in the case of normal faults, were derived almost 15 years ago using a dataset of US earthquakes. We collected additional case histories worldwide, for a total of 21 earthquakes, and calculated the conditional probability of distributed faulting as a function of distance from the principal fault. We found no clear dependency on the magnitude nor the time of occurrence of the earthquakes, but our data consistently show a higher probability of rupture when compared with the scaling relations currently adopted in engineering practice. We derive updated empirical regressions and show that the results are strongly conditioned by the averaging of earthquakes effectively generating distributed faulting at a given distance and those which did not generate faulting; thus, we introduce a more conservative scenario that can be included in a logic tree approach to consider the full spectrum of potential ruptures. Our results can be applied in the framework of probabilistic assessment of fault displacement hazard.

Electrical resistivity tomographic detection of the hidden Thakek fault, Northeast Thailand

ABSTRACTThe Khorat plateau covers most of northeast Thailand along with parts of southern Laos. Recent earthquake hazard assessment and other studies of the regional Thakek fault have rarely appeared in publications and reports. The Thakek fault zone in the Laos region includes three main principal fault planes, but it rapidly fades out when the fault passes through Thailand. A geophysical hidden fault study in the Khorat plateau is herein performed in Bung Kan and Nakhon Phanom provinces, Thailand, where the surface expression is difficult to document through conventional geological surveys. Some fault traces have been identified by satellite image interpretation and digital elevation model analysis. Geological field investigation and electrical resistivity tomography surveys were carried out in order to determine an optimal site for trenching. The electrical resistivity tomography results show zones of high and low resistivity interpreted as coarse sediment layers and mudstone, respectively. The maximum vertical fault offset occurring along the fault trace interpreted from the three‐dimensional electrical resistivity tomographic slices is 40 ± 5 m. Based on the electrical resistivity tomography results, a trench was excavated to directly investigate patterns of faulting and deformation. Based on structural geological observations at the trench wall, indications of faulting have been tentatively identified. In particular, faulting appears to have displaced mudstone over Quaternary sediments with vertical offsets >3.5 m cutting the youngest unit. The result has implications for seismic hazard assessment in northeast Thailand.

Three-dimensional geometry and growth of a basement-involved fault network developed during multiphase extension, Enderby Terrace, North West Shelf of Australia

Abstract Basement fault reactivation, and the growth, interaction, and linkage with new fault segments are fundamentally three-dimensional and critical for understanding the evolution of fault network development in sedimentary basins. This paper analyzes the evolution of a complex, basement-involved extensional fault network on the Enderby Terrace on the eastern margin of the Dampier sub-basin, North West Shelf of Australia. A high-resolution, depth-converted, 3-D seismic reflection data volume is used to show that multiphase, oblique extensional reactivation of basement-involved faults controlled the development of the fault network in the overlying strata. Reactivation of the pre-existing faults initially led to the formation of overlying, en échelon Late Triassic–Middle Jurassic fault segments that, as WNW-directed rifting progressed on the margin, linked by breaching of relay zones to form two intersecting fault systems (F1 and F2–F4). Further reactivation in the latest Jurassic–Early Cretaceous (NNW-SSE extension) produced an additional set of en échelon fault arrays in the cover strata. The final fault network consists of main or principal faults and subordinate or splay faults, together with branch lines that link the various components. Our study shows that breaching of relay ramps and/or vertical linkages produces vertical and horizontal branch lines giving complex final fault geometries. We find that repeated activity of the basement-involved faults tends to form continuous and planar fault architectures that favor displacement transfer between the main constituent segments along strike and with depth.

A case for limited global contraction of Mercury

Mercury is a one-plate planet that has experienced significant radial contraction primarily driven by interior cooling. In some previous studies aimed at estimating the total magnitude of contraction, numerous faults are assigned to positive relief landforms, many without evidence of origin by deformation, resulting in estimates of planetary radius reduction as large as 7 km. Here we use high-incidence angle image mosaics and topography from the MESSENGER mission to map Mercury’s contractional landforms. Each landform is assigned a single, principal fault, resulting in an amount of contractional strain equivalent to a radius change of no more than 1 to 2 km. A small radius change since the end of heavy bombardment is consistent with Mercury’s long-lived magnetic field and evidence of recent tectonic activity. It is concluded that the retention of interior heat and a lower degree of contraction may be facilitated by the insulating effect of a thick megaregolith.

A wind turbine frequent principal fault detection and localization approach with imbalanced data using an improved synthetic oversampling technique

Frequent principal fault detection and localization (FPFDL), as a new problem of fault diagnosis of the wind turbine system in practice, has gained a growing concern in wind power industries. The knowledge-based fault diagnosis method with historical wind buffer data is a feasible way to solve this problem. However, due to the uncertainty of the principal fault and the incompletion of the practical data, the wind buffer data used is imbalanced, inadequate, and unfixed-length, which leads to higher misclassification rates for the minority classes by traditional machine learning methods. To overcome these challenges, a novel FPFDL approach is proposed in this paper. Firstly, we design an improved oversampling algorithm to generate and develop the balanced dataset based on the imbalanced dataset of unfixed-length. This algorithm combines the dependent wild bootstrap oversampling and the modified synthetic minority oversampling technique. So, it can consider the temporal dependence and the relationship between the samples during data oversampling. Secondly, we introduce the one-dimensional convolutional neural networks to achieve automatic high-level local feature extraction and fault identification. Finally, the experimental results of seven cases using the datasets collected from two real wind farms in China validate our proposed approach’s effectiveness and robustness with imbalanced wind buffer data of unfixed-length.

Probability of Occurrence and Displacement Regression of Distributed Surface Rupturing for Reverse Earthquakes

Probabilistic fault displacement hazard analysis provides a systematic approach to estimate the likelihood of occurrence and expected amount of surface displacement during an earthquake on-fault (principal fault rupturing) and off-fault (distributed rupturing). The methodology is based on for key parameters describing the probability of occurrence and the spatial distribution of the displacement both on and off-fault. In this work we concentrate on off-fault rupturing, and develop an original probability model for the occurrence of distributed ruptures and for the expected displacement distribution based on the compilation and reappraisal of surface ruptures from 15 historical crustal earthquakes of reverse kinematics, with magnitudes ranging from Mw 4.9 to Mw 7.9. We introduce a new ranking scheme to distinguish principal faults (rank 1) from simple distributed ruptures (rank 2), bending-moment (rank 21) and flexural-slip (rank 22) and triggered faulting (rank 3). We then used the rank 2 distributed ruptures with distances from the principal fault ranging from 5 m to 1500 m. To minimize bias due to the incomplete nature of the database, we propose a ‘slicing ' approach as an alternative to the 'gridding' approach. The 'slicing' approach is then combined with MonteCarlo simulations to model the dependence of the probability of occurrence and exceedance with the dimensions and position of the site of interest with respect to the principal fault, both along and across strike. We applied the probability model to a case-study in Finland to illustrate the applicability of the method given the limited extend of the available dataset. We finally suggest that probabilistic fault displacement hazard model will benefit by evaluating spatial distribution of distributed rupture in the light of spatial completeness of the input data, structural complexity and physics observables of the causative fault.